This invention relates to the nuclear-reactor art. In this art, the nuclear-reactor plants are provided with spent-fuel storage facilities, specifically a spent-fuel storage pool having racks for storing spent fuel under a substantial depth of water. This invention relates particularly to the storage of spent fuel and to racks for storing such fuel.
Nuclear plants produce large quantities of spent fuel. In the past, it was contemplated that the spent fuel would be reprocessed to provide fissionable uranium and plutonium as a renewed fuel. But the Nuclear NonProliferation Treaty, to which the United States is a party, has been interpreted to bar the reprocessing of fuel inasmuch as plutonium, a product of the reprocessing, is a weapons material. It then becomes necessary to cope with the problem of safely handling the spent fuel which continuously emerges from nuclear plants. It is an object of this invention to provide an effective solution for this problem. It is contemplated that the spent fuel must be stored for a number of years before it can safely be disposed of as nuclear waste. The solution of the spent-fuel problem deals with the storage of the spent fuel during these years.
Spent fuel retains a measure of reactivity, i.e., neutron emissivity, which is appreciable but is insufficient for economic use in a reactor. It is then necessary that the spent fuel be stored in such a way that the mass stored does not become critical. In refueling a reactor, the fuel assemblies in specified areas of the reactor are replaced at intervals of several years. The residual reactivity of the removed fuel assemblies throughout each area of the refueling is not uniform. It is then necessary, in the storage of spent fuel, to preclude nuclear criticality by reason of the presence of fuel assemblies having high residual reactivity. In accordance with the teachings of the prior art, such criticality is precluded by providing racks whose cells are appropriately spaced. In addition, quantities of neutron-absorbing material or poison can be provided in the cells of the racks in which the spent fuel is stored. The first of these expedients requires that the volume occupied by racks be unreasonably large. The second expedient introduces a high cost factor. It is an object of this invention to provide for the storing of spent fuel in a way that shall be economic both financially and with respect to volume.
This invention relates to storage of spent fuel in a rack of uniformly spaced cells. The cell dimensions and the actual center-to-center (CTC) spacing is set to accommodate the types of fuel which are to be stored in the rack. Each fuel-storage pool is subdivided into two regions, herein designated Region 1 and Region 2. Region 1 is the smaller region and is reserved for off-core loading; i.e., for temporary loading of spent-fuel assemblies, regardless of burn-up, as they are removed from the reactor. Typically, Region 1 may serve to load about 200 fuel assemblies at a low fuel-assembly density using a fraction, usually half of the available storage locations. Region 2 is reserved for storing, for the required long time interval, the assemblies from Region 1 which have been found to have sustained at least a minimum predetermined burn-up. As used in this application, the expression "storage location" means generally a location where fuel assemblies are stored. Specifically, this term comprehends cells or locations or bodies which may be formed between a plurality of cells.
In Region 1 the storage locations are in a checkerboard pattern in a honeycomb type structure. One set of storage locations in this honeycomb are capped to prevent the insertion of fuel assemblies, while the other set is uncapped. In a typical checkerboard pattern, a square of one color, for example, black alternates with a square of another color, for example, red, so that the total area of black squares is equal to the total area of red squares. Typically, the checkerboard pattern or honeycomb structure of Region 1 in the practice of this invention may take this form. However, a structure in which a number of contiguous cells are capped in uniformly spaced areas of the cell surface may also be provided. This structure is necessary to reduce the possibility of criticality. The reference in this application to a "generally checkerboard" pattern is intended to cover this structure as well as one simulating an actual checkerboard. It is necessary that the reactivity K.sub.eff of the spent fuel assemblies in Region 1 be maintained at less than or equal to 0.95. For this purpose, the cells may be provided with neutron-absorbing material. The water in which the rack is immersed may be maintained adequately neutron absorbent by the solution therein of a boron compound or the compound of another neutron-absorbing element.
The spent-fuel assemblies in Region 1 are surveyed through administrative control to determine burn-up. Those assemblies that have sustained the required burn-up are transferred to Region 2 where they remain until their radioactivity is reduced to a magnitude permitting removal from the pool and other disposal.
It is desirable that, in the use of a spent-fuel storage pool, the option be afforded to include neutron-absorbing poison in the rack either before the rack is installed or after it is installed. In spent-fuel facilities in accordance with the teachings of the prior art, such racks are not provided. If neutron poison is necessary, it must be included before the rack is installed in the pool; it cannot readily be included in the rack after the rack is installed. Another deficiency of prior art spent-fuel facilities is that the CTC spacing between storage cells is maintained by grids or like components. This is an undesirable and space-consuming complication.
It is an object of this invention to overcome the above-described disadvantages and drawbacks of the prior art and to provide a method of making a spent-fuel storage rack which shall occupy a minimum volume and shall afford the option of including the nuclear poison either before or, readily, after the rack is installed in the pool and in whose use the CTC spacing shall be reliably maintained with economy in space and without spacing grids.